Sounds like something electrical heating up. If the smell is inside the van one common place to check is the climate control panel connection. The connector is known to melt with heavy blower motor use. Another would be the blower resistor block up behind the glovebox but only if you have the manual climate system. There was a recall on 02s (???) for water driping into the radio causing failures. Has that been taken care of? Are you seeing any system failures or querks? If you start smelling rotten eggs your cat converter could be over heating.
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96 Gr Caravan LE 3.8, 212K miles
07 Fusion / 06 RAV 4 / 06 Accord